Chemistry Paper 2,Nov/Dec 2010  
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General Comments

Question 7

(a) (i) State the law if multiple proportion

(ii) A metal M forms two chlorides P and Q. On analysis, 2.00g of P was found to contain 0.69 g of M and 10.00 g of Q was found to contain 4.41 g of M.

          I.Calculate the mass of chlorine which combined with 1.00g M to form P and Q respectively
           II.         Deduce whether the law of multiple proportions is obeyed.
            III.        Write the correct formula of P and Q. [ 13 mark s]

(b) Give the IUPAC name of each of the following substances:
          (i)         K4Fe(CN)6;
          (ii)        Na2CO3H2O [2 marks]

         (c)     (i)    What are salts?
                 (ii) State one factor that can influence the method used for the preparation of a salt.
                (iii)          Classify the following salts into acid, basic, double and complex salts:

I.       Na2Zn(OH)
II.      Zn(OH)CI;
III. (NH4)2Fe(S04)2.6H20 .
Write a two-step chemical equation to show the prepration of sodium hydrogen
trioxocarbonate (IV) from sodium hydroxide.                                      [10 marks]



The question was attempted by few candidates and the performance was fair.

In(a)(i), candidates correctly stated law of multiple proportions thus:

If two elements, A and B combine to form more than one compound, then the various masses of one element A which combine separately with a fixed mass of the other element B are in simple ratio.

In(a)(ii), most candidates failed to correctly calculate and verify the law of multiple proportion and hence, write the correct formula of P and Q. The expected answers from candidates were as follows:

              = 1.5 x 2                             1.0 x 2
           =        3           :                           2                                                                       

Since the masses of chlorine which combine with LOg of M in P and Q are in simple ratio 3:2 then the law of multiple proportion is obeyed

            III.      P - MCl3
                    Q - MCl2

In part (b), candidates has shallow knowledge of the IUPAC nomenclature hence could not name each of the substances in (i) and (ii). The expected answers from candidates were:
                                  (i)         Potassium hexacyanoferrate(II);
                                 (ii)       Sodium trioxocarbonate (IV) monohydrate.

In(c)(i), candidates correctly stated that salts are compounds formed when all or part of the ionizable hydrogen of an acid are replaced by metallic or ammonium ions/when all part of the OW
ions of a base are replaced by non metallic ions.

In (cHii), candidates knew that either solubility in water (solubility) or stability to heat as a factor that influences the method used for the preparation of a salt.

In (cHiii), only few candidates could classify the given salts accordingly. The required answers from candidates were as follows:

                                                    I.       Na2Zn(OH) - complex salt,
                                                    II.      Zn(OH)CI - basic salt;
                                                    III. (NH4)2Fe(S04)2.6H20.- double salt.

In(c)(iv), most candidates could not write a two-step equation to show the preparation of sodium hydrogen trioxocarbonate(lV) from sodium hydroxide. The expected answers from candidates

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